The instant invention relates to a process for the polymerization of ethylene alone or with comonomers and/or telogens (modifiers) at elevated temperatures and pressures in an elongated tubular reactor wherein relatively high conversions are obtained without loss of optical or physical product quality.
The polymerization of ethylene to solid polyethylene in an elongated tubular reactor at elevated temperatures and pressures in the presence of free radical or free oxygen producing initiators that decompose at or below the polymerization temperature to generate free radicals is well known in the art. Various types of tubular reactor systems have been developed and are in commercial use. One of the more basic systems encompasses feeding a pressurized stream consisting of ethylene, initiator and optionally a modifier into one end of a tubular reactor consisting of one reaction zone and one cooling zone. Owing to the exothermic nature of the reaction, the temperature increases as the reaction proceeds to a maximum or peak temperature and considerable heat is evolved. Heat control methods must be employed to prevent violent run away reactions and explosions as well as to increase conversions. One method for achieving heat control involves the use of a long tubular reactor wherein some measure of temperature control is obtained from the relatively large ratio of heat dissipating surface to reactor volume. Thus, heat control is also accomplished by providing the reactor with a cooling jacket.
It has also been discovered that effective control of the polymerization reaction can be maintained together with an increase in conversion if instead of introducing ethylene at one point in the reactor, it is injected as sidestreams at one or more additional points downstream in the reaction tube. The injected ethylene is made to serve both as a coolant and as a monomer for further polymerization. See U.S. Pat. No. 3,725,378.
Another process using multiple sidestreams which is incorporated herein by reference is disclosed in U.S. Pat. No. 3,628,918. This patent discloses three reaction zones, three cooling zones and two monomer sidestreams; one introduced prior to the second reaction zone and the other introduced prior to the third reaction zone in a tubular reactor having a larger diameter for the cooling zones than that of the reaction zones.
Generally, all of the conventional polymerization systems attempt to achieve the highest level of conversion without loss of optical and physical properties of the resulting polymer. For example, the two reaction zone and three reaction zone processes discussed above can achieve about 20 to 25 percent conversion. It has heretofore been assumed and observed by those skilled in the art that when conversion levels about 25 percent are achieved, both optical and physical properties rapidly deteriorate regardless of the pressure and temperature histories achieved.
The quality of polyethylene particularly haze is affected by a number of mechanisms. Haze increases with increasing high molecular weight components. By maintaining a high flow number and preferably a flow number greater than 3.3 ft.sup.2 /sec. one obtains the greatest percent of turbulent flow and thereby minimizes long straight chains which are formed dominantly during the laminar flow. On the other hand increase in reaction zone average pressure improves haze by decreasing the amount of long chain branching thereby decreasing the high molecular weight component in the molecular weight distribution. An increase in reaction zone average or peak temperatures increases long chain branching of the polymerization reaction. The increased high molecular weight component in the molecular weight distribution results in increased haze. Additionally, the increase in polymer concentration obtained through higher conversions was known to generally cause an increase in the opportunity for the long chain branches to form on existing polymer molecules. Thus, it has generally been accepted that high conversions lead to greater long chain branches and hence reduce haze quality, i.e., higher haze.